, the buyer can transfer foreign currency for the purchase from an overseas bank account into the seller’s Thai bank account for the Thai bank to issue a Foreign Exchange Transaction Form…..of course this will depend on the level of trust.
You need a Thai work visa to be a real estate agent (and any other jobs requiring a physical presence in Thailand). There are lots of examples of people who have otherwise gotten in trouble.
Singapore immigration will not stop you from entering and leaving the same day…..but Thai immigration might look at your travel history before granting you entry.